The present invention relates to new solid solutions of quinacridones and to their use for pigmentation of, for example, fibers, plastics, coatings, and printing inks. In particular, this invention relates to violet compositions containing 95% to 5% by weight of 2,9-dimenthoxyquinacridone and 5% to 95% by weight of 2,9-dichloroquinacridone mixed under certain conditions with each other to form violet solid solutions that are quite different from both physical mixtures of such compounds and from the individual compounds themselves.
The term "solid solution" as used herein describes a well recognized physical property of certain solid substances. In a solid solution, the molecules of the components enter is not the same crystal lattice, usually (but not always) the crystal lattice characteristic of one of the components. The X-ray pattern of the resulting crystalline solid is characteristic and can be clearly differentiated from the pattern of a physical mixture of the same components in the same proportion. Thus, the X-ray peaks of each component of a physical mixture can be distinguished, whereas a characteristic of a solid solution is the disappearance and/or shift of many of these peaks.
Solid solutions of quinacridone pigments have been reported to provided advantageous color properties. For example, U.S. Pat. Nos. 3,160,510, 3,298,847, and 4,895,949 disclose solid solutions of ring-substituted quinacridone pigments said to have improved tinctorial and lightfastness properties. Suitable methods for preparing such solid solutions of quinacridones are also described, for example, in U.S. Pat. No. 3,607,336 and British Patents 896,916 and 955,854. Although quinacridones having a variety of ring substituents, including methoxy and chloro substituents, are disclosed, these patents do not exemplify solid solutions of 2,9-dimethoxyquinacridone and 2,9-dichloroquinacridone and do not suggest the special advantages of such solid solutions of the present invention. Example 13F of U.S. Pat. No. 3,160,510, for example, discloses a solid solution of 90 parts 2,9-dimethoxyquinacridone and 10 parts 2,9-dimethylquinacridone. Comparison of the color properties of this solid solution with the corresponding physical mixture shows that the solid solution exhibits less chroma (an indication of brightness) than the physical mixture. Low chroma is perceived as a dullness feature and is not desirable. In contrast, the solid solutions of the present invention are typically higher in chroma than the physical mixtures.
Two important properties of the present invention render the new products particularly useful as pigments. First, in contrast to simple physical mixtures wherein the color is usually a direct function of the additive effects of the two components, the new solid solutions of the present invention show unexpectedly and unpredictably advantageous tinctorial properties.
A second valuable property of the invention is an enhancement of lightfastness, which frequently accompanies the formation of solid solution. In physical mixtures of two pigments, the components show their individual behaviors upon exposure to light, often resulting in marked changes of hue as one pigment component fades more than the other. In contrast, however, the solid solutions of the present invention behave as a single substance with respect to hue stability.
The solid solutions of 2,9-dimethoxyquinacridone and 2,9-dichloroquinacridone according to the invention are very clean, very bright, violet, non-bleeding pigments exhibiting a deep, transparent masstone. The cleanness, brightness, blue hue, and excellent lightfastness of the solid solution pigments of the invention allow them to be used in styling with other pigment types in a manner that cannot currently be accomplished commercially. Where optimum bluish hue is desired, solid solutions in which the content of 2,9-dimethoxyquinacridone is from 50 to 80% by weight are particularly preferred. Physical mixtures of the individual quinacridones lack the brightness, transparency, strength, and performance properties of the solid solutions.